Corrin Syntheses. Part VI

Abstract

Step of the Corrin Synthesis via A ! D Ring Closure D: The Photochemical A/D-Secocorrin ! Corrin Cycloisomerization: Exploratory Studies on its Mechanism.Chapter A describes the background of the two topics discussed in Chapts. B and C within the general context of the project Corrin Syntheses.Chapter B deals with the development and application of C,C-bond formation by sulfide contraction as a general method for constructing the chromophore system in corrin ligands. The method had been developed in the context of the task of joining precursors of ring B and ring C to the B/C component in the collaborative Harvard/ ETH project of the synthesis of vitamin B 12 . Conceptually, the methods central process corresponds to an intramolecularization of an otherwise intermolecular C,C-bond formation by a (thio)imido-ester/enamine (enamide) condensation. The procedure starts by a preliminary intermolecular oxidative coupling of the S-atom of a thiolactam group with the electrophilic C-center of an enamine or enamide group to give an imidoyl-vinyl-sulfide intermediate, which, in a follow-up step, is intramolecularly contracted to a vinylogous amidine system with concomitant transfer of the S-atom to an external thiophile (cf. Fig. 3). The method has been successfully applied to the construction of A/D-secocorrin model substrates in three model series by three different strategies of connecting the four peripheral rings A, B, C and D. These strategies differ in the sequence by which these four rings are joined together (cf. Figs. 5 and 6, 10 and 11, and 13). A/D-Secocorrin complexes obtained by a B þ C ! BC þ A ! ABC þ D ! ABCD strategy served as main model substrates for the exploration of the photochemical A/D-secocorrin ! corrin cycloisomerization, whereas an A/Dsecocorrin chloro-Zn II complex constructed by a BC þ D ! BCD þ A ! ABCD strategy was the target of a model study pursued as a pacemaker synthesis for the construction of the chromophore system in the photochemical variant of the vitamin B 12 synthesis.In Chapt. C, the targeted discovery and some of the applications of the photochemical A/D-secocorrin ! corrin cycloisomerization are described. The concept of this novel process emerged for a corrin synthesis in which all four rings are derived from one single starting material, reflecting the hidden structural symmetry of the vitamin B 12 molecule. The reaction was studied in three A/D-secocorrin model systems (cf. Chapt. A), the A/D-secocorrin model in the 15-cyano-2,2,7,7,12.12-hexamethyl-1methylidene series being the one that served as the main substrate in the exploration of the reaction. The A ! D ring closure proceeded smoothly under the influence of visible light and under strict exclusion of O 2 in yields of over 90% in A/D-secocorrin complexes of the filled-shell metal ions Zn II , Cd II , Mg II , Li I , and (tentatively) also Ca II and Na I , but not for complexes of the first-row transition metals such as Ni II , Cu II , and (tentatively) Co III , Mn II , and Fe II . The reaction also occurs -more slowly...

Copper-Catalyzed Coupling of Thioamides and Donor/Acceptor-Substituted Carbenoids: Synthesis of Enamino Esters and Enaminones

“…Finally, BF 3 ·OEt was attempted in the reaction (entry 17) as it has shown to be one of the best Lewis acids in many Lewis-acid promoted reactions of diazo compounds 40−46 and has been used to promote sulfur extrusions in the ESR reaction. 7,28,29 The reaction failed to give the enamino ester and within 10 min. all 2a dimerized at room temperature.…”

Steric and Electronic Factors Influence Regio‐isomeric Thiazole Formations Between Substituted Benzothioamides and Ethyl Bromopyruvate

“…Secondly, the electron‐withdrawing chloro‐substitutions at the ortho ‐positions of the phenyl group might also destabilize the developing carbocation through inductive effect. As a result, the excess pyridine can deprotonate the α‐proton of the ketone in 1F and the resulting enolate can add to the thio‐imidate intramolecularly to give thiirane 1G , in a manner highly reminiscent of the initial steps in Eschenmoser's sulfide contraction chemistry . The opening of thiirane in intermediate 1G leads to intermediate 1H in which the thiol can undergo nucleophilic addition to the adjacent ketone that results in thiirane 1I .…”

Zinc Substitution of Cobalt in Vitamin B12: Zincobyric acid and Zincobalamin as Luminescent Structural B12‐Mimics

“…The phosphorescence of photo‐excited Znby decayed with a lifetime of 13±1 msec at 77 K. Znby sensitized the formation of 1 O 2 with a quantum yield Φ Δ =0.70. The Zn‐corrin ZnCor emitted fluorescence with a maximum at 573 nm (Φ f =0.09) at room temperature in EtOH, and was an efficient triplet sensitizer in the legendary photo‐induced A/D‐secocorrin to corrin cycloisomerization …”

“…The phosphorescence of photo-excited Znby decayed with al ifetime of 13 AE 1msec at 77 K. Znby sensitized the formation of 1 O 2 with aquantum yield F D = 0.70. TheZ n-corrin ZnCor [6a] emitted fluorescence with am aximum at 573 nm (F f = 0.09) at room temperature in EtOH, [16] and was an efficient triplet sensitizer in the legendary photo-induced A/D-secocorrin to corrin cycloisomerization. [4a, 7, 16] Top: X-ray crystal structure of Znby in two projections (color coding:red = corrin core carbons and oxygens;g reen = other carbons; blue = nitrogens;gray = Zn).…”